Constant voltage transformers

ABSTRACT

A constant voltage regulators in which the output current is limited by a series impedance and the output voltage is shorted for the remainder of each half cycle by the operation of a solid state switching device whose operation is initiated when the time integral of the output voltage taken with zero initial conditions at the start of each half cycle reaches a predetermined magnitude.

United States Patent Tolworthy et al. 1 1 Jan. 30, 1973 54 CONSTANT VOLTAGE [56] References Cited TRANSFORMERS UNITED STATES PATENTS [75 I Inventors E i; iz gfi g g 32' 3,355,650 ll/l967 Tolmie .....323/11 l d g 3,435,248 3/1969 Geis .323/22 sc x Straffordshlrer Eng 3.211929 10/1965 Princset al. ..323/11 x [73] Assignee: Thorn Automation Limited, Rugely, 35761443 W971 England 3,571,696 3/l97l 3,l46,392 8/l964 [22] Filed: Jan. 22,1971 v I 2 APPL 6 10 713 Primary ExaminerGerald Goldberg I Attorney-Hurvitz & Rose [30] Foreign Application Priority Data [57] ABSTRACT Jan. 28, 1970 1 Great Britain ..4,l65/70 A constant vohage regulators in which the Output rent is limited by a series impedance and the output [5.2] Cl i i/ g g d voltage is shorted for the remainder of each half cycle [51'] Int Cl 6 1/44 by the operation of a solid state switching device [58] Field 40 81 whose operation is initiated when the time integral of 323/DlG. 1;?107/246, 252 8,252 P; 3l7/l6 the output voltage taken with zero initial conditions at the start of each half cycle reaches a predetermined magnitude.

5 Claims, 3 Drawing Figures PAT NTEDJTTTBO I975 I 3,714,546

T5 I 4 B 4 9 B 0 4 FA D1 Eli o 0 R D5 7 RV1 I R R 1 6 T 3 g T3 3 20! R2 r r ZDQ 203 F C2 (31L TT R5 3 Dz. '12 f T Inventors I ROBERT THEODORE TOLVJRTHY & JOHN HAROLD LEACH y QM Attorneys CONSTANT VOLTAGE TRANSFORMERS This invention relates to constant voltage regulators such as are often known as constant voltage transformers.

A known form constant voltage REGULATOR essentially comprises a series current limiting impedance (usually an inductor) and a parallel saturable inductor which is so arranged as to saturate when the time integral of the voltage across it has reached a predetermined value thereby shorting together the output terminals of the transformer. These functions are usually incorporated in a specially constructed transformer. Two of the many disadvantages inherent in such a constant voltage regulator are a) it is often heavier than is desirable due to the inherent weight of the essential saturating inductor and b) particularly at high frequency, for example 400 cycles, it is undesirably inefficient due to hysterises and eddy current losses in the inductors core.

In a constant voltage regulator in accordance with the invention the output current is limited by a series impedance and the output voltage is shorted for the remainder of each half cycle by the operation of a solid state switching device whose operation is initiated when the time integral of the output voltage taken with zero initial conditions at the start of each half cycle, reaches a predetermined magnitude. The constant voltage function is thus separated from the transforming function. An advantage of such a constant voltage regulator lies in the saving in weight which may be obtained by substituting a solid state switching device for the saturating inductor of known forms of a constant voltage regulator. Further there are no hysterises or eddy current losses associated with such a solid state switching device which may pass substantially no leakage current when in its off condition and which may absorb substantially no power when in its on condition.

The operation of the solid state switching device is preferably initiated by firing means arranged to full wave rectify the output voltage, the rectified voltage being integrated, from a predetermined initial condition of the start of each half cycle, by a serially connected resistor and capacitor, threshold dependant solid state trigger being so connected as to initiate the operation of the solid state switching device when the voltage across the capacitor reaches a predetermined value. This particular construction is advantageous in that since the output voltage is rectified and since the integration is carried out from a predetermined initial condition at the start of each half cycle the circuit has no memory of the preceding half cycle. In contrast, in the above mentioned known forms of constant voltage regulators, after saturating on one half cycle the saturating inductor retains a high level of flux in its core and if the supply is then cut off and reinitiated in the same polarity as that of the last half cycle before it was switched off the inductor saturates almost immediately with the result that a large current is drawn from the supply dictating unsafe fuse ratings or frequent fuse replacement. Further it will be appreciated that the constant voltage regulator of the invention may be used with a uni-directional periodic supply. In contrast the known forms of constant voltage transformer may not be so used since the saturating inductor would remain permanently in the saturated condition.

The solid state switching device preferably comprises a triac or two controlled rectifiers connected antiparallel. An advantage of this arrangement is that the firing means need only provide the firing pulse it not being necessary to maintain the signal to the solid state switching device in order to keep it conducting up to the end of each half cycle.

The firing means preferably includes active elements which have no primary power source other than the rectified voltage.

Further the firing means preferably include means to ensure the capacitor is in a discharged state at the end of each half cycle, the predetermined initial condition at the start of each half cycle in this case being zero.

One embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which:

FIG. 1 is a general circuit diagram of a constant voltage regulator in accordance with the invention;

FIG. 2 is a more detailed circuit drawing of the solid state switching device and its firing means;

FIG. 3 is a general circuit diagram of a modified constant voltage regulator in accordance with the invention.

Referring firstly to FIG. 1 it will be seen that the constant voltage regulator has input terminals 2 for connection to an alternating voltage supply or to a periodic voltage supply, and output terminals for connection to a load. Serially connected between the input and the output is an impedance 6, in this case an inductor. Connected between the two output terminals is a solid state switching device T in this case a triac. Connected in parallel with the triac T and to its gate is a firing circuit 8. In operation the output current is limited by the series inductor 6 and the firing circuit 8 acts to fire the triac so that it shorts together the output terminals for the remainder of each half cycle when the time integral of the output voltage, taken with zero initial conditions at the start of each half cycle, reaches a predetermined magnitude. Thus the rectified means of the output voltage appearing at the terminals 4 is substantially independant of variations in the voltage supply at the end of terminals 2 of variations in the load impedance connected to terminals 4.

Referring now to FIG. 2 it will be seen that the voltage appearing across terminals AB of the firing circuit 8 (i.e. the output voltage) is full wave rectified by diodes D D D and D, so providing a positive and a negative supply rail 10 and 12 respectively. The rectified voltage is integrated by a resistor capacitor arrangement RV,, R and C the rate at which capacitor C, charges being adjustable by means of the variable resistor RV,. The voltage across capacitor C thus represents the time integral of the voltage between the terminals A and B. The arrangement is such that the voltage across the capacitor which represents the predetermined magnitude of the predetermined time integral of the output voltage is small compared with the voltage between the terminals A and B. This voltage is determined by a serially connected resistor and Zener diode chain R and ZD to the junction of which is connected the emitter of an NP.N. transistor whose base is connected to the junction between capacitor C and its series resistance R,.

When capacitor C has charged to a sufficiently high voltage transistor T conducts causing transistors T and T of a regenerative switch arrangement rapidly to conduct. Initially the current through the regenerative switch T T passes through capacitor C However, when the voltage across capacitor C has built up sufficiently the current is conducted through a Zener diode ZD which is connected in parallel with capacitor C The said current is divided between resistor R and a path through a primary winding of a transformer TR and through the base emitter junction of a transistor T The step of current so passing through the primary winding of transformer TR induces in its secondary winding a voltage pulse which fires the triac T through A resistor R The current also acts to cause transistor T to conduct so discharging capacitor C,.

In order to ensure that there is a suffiently low impedance voltage source for the above described operations to take place, even towards the end of a half cycle when the magnitude of the rectified voltage is diminishing, a capacitor C is connected across the regenerative switch arrangement T T and is charged during each half cycle from a potential divider R and R and diode D to a voltage limited by a Zener diode ZD At the end of each half cycle the triac T ceases to conduct and must be retired during the next half cycle.

In order to ensure that the regenerative switch T T turns off at the end of each half cycle capacitor C is provided with a discharge path through the emitter to base junction or transistor T in parallel with resistor R and through diode D, so that when the voltage between the terminals A and B has reduced sufficiently for diode D to be forward biased the capacitor C begins to discharge. When the capacitor C is discharged to a voltage which is lower than the voltage stored on the capacitor C the regenerative switch T T turns off.

Further if due to an exceptionally low output voltage the capacitor C, does not charge sufficiently to trigger triac T when the capacitor C begins to discharge through the resistor R and the diode D, the voltage drop across the resistor R so biases the base of transistor T;, as to cause the regenerative switch T T to conduct so firing triac T and causing transistor T to conduct and thus discharge capacitor C,. When the capacitor C; has discharged to a lower voltage than the voltage stored on the capacitor C regenerative switch T T again cuts off. The capacitor C is thus in a discharge state at the beginning of each half cycle.

It will be appreciated that the constant voltage regulator described will equally operate on a uni-directional periodic supply as on alternating supplies providing that the periodic supply reduces to zero potential in each period long enough for T to turn off.

An advantage of the above arrangement which will be apparent is that by adjusting resistor RUl so as to vary the charging rate of the capacitor C the output voltage of the transformer may easily be adjusted.

In the modified arrangement shown in FIG. 3, the triac is replaced by two control rectiflers T a and T b arranged in anti-parallel.

We claim:

1. A constant voltage regulator for a cyclic voltage supply comprising:

input terminals;

outputterminals; a current-limiting impedance connected in series between an input terminal and an output terminal; solid state latching switching device connected across the output terminals and initiable to short the output terminals together, said switching device being of the type which unlatches each time the short circuit current therethrough falls to substantially zero;

re-settable integrating means for generating a signal dependent upon the time integral of the voltage across said output terminals; threshold-dependent initiating means for initiating said switching device each time the magnitude of said signal exceeds a predetermined value; and

discharge path means, including actuable switching means, for re-setting said integrating means each time the voltage across said output terminals falls to substantially zero, said discharge path means, when said switching means is actuated, having an impedance which resets said integrating means to the same initial condition each time said integrating means is reset.

2. A constant voltage regulator as claimed in claim 1 wherein said switching device comprises a triac.

3. A constant voltage regulator as claimed in claim 1 wherein said switching device comprises two controlled rectifiers connected anti-parallel.

4. A constant voltage regulator as claimed in claim 1 including full wave rectifying means connected across the output terminals, wherein the integrating means is so connected as to integrate the rectified output of said rectifying means.

5. A constant voltage regulator as claimed in claim 4 wherein the integrating means and the initiating means comprise active elements which have no primary power source other than the rectified voltage.

* i w a: a: 

1. A constant voltage regulator for a cyclic voltage supply comprising: input terminals; output terminals; a current-limiting impedance connected in series between an input terminal and an output terminal; a solid state latching switching device connected across the output terminals and initiable to short the output terminals together, said switching device being of the type which unlatches each time the short circuit current therethrough falls to substantially zero; re-settable integrating means for generating a signal dependent upon the time integral of the voltage across said output terminals; threshold-dependent initiating means for initiating said switching device each time the magnitude of said signal exceeds a predetermined value; and discharge path means, including actuable switching means, for re-setting said integrating means each time the voltage across said output terminals falls to substantially zero, said discharge path means, when said switching means is actuated, having an impedance which resets said integrating means to the same initial condition each time said integrating means is reset.
 1. A constant voltage regulator for a cyclic voltage supply comprising: input terminals; output terminals; a current-limiting impedance connected in series between an input terminal and an output terminal; a solid state latching switching device connected across the output terminals and initiable to short the output terminals together, said switching device being of the type which unlatches each time the short circuit current therethrough falls to substantially zero; re-settable integrating means for generating a signal dependent upon the time integral of the voltage across said output terminals; threshold-dependent initiating means for initiating said switching device each time the magnitude of said signal exceeds a predetermined value; and discharge path means, including actuable switching means, for re-setting said integrating means each time the voltage across said output terminals falls to substantially zero, said discharge path means, when said switching means is actuated, having an impedance which resets said integrating means to the same initial condition each time said integrating means is reset.
 2. A constant voltage regulator as claimed in claim 1 wherein said switching device comprises a triac.
 3. A constant voltage regulator as claimed in claim 1 wherein said switching device comprises two controlled rectifiers connected anti-parallel.
 4. A constant voltage regulator as claimed in claim 1 including full wave rectifying means connected across the output terminals, wherein the integrating means is so connected as to integrate the rectified output of said rectifying means. 